Sexual transmission through the genital tract or rectal mucosa is the most common route for acquiring new HIV infections and accounted for ~70% of the 2.7 million people worldwide who became newly infected in 2007. A cure or effective vaccine that would contain the global spread of this epidemic is not expected in the near term, and new HIV infections continue to outpace advances made in treatment with antiretroviral drugs. There is consequently an urgent need to develop agents that can be applied topically to mucosal surfaces to prevent the sexual transmission of HIV. However, several large-scale clinical trials testing the efficacy of agents that disrupt the integrity of the viral envelope (detergents) or prevent adsorption or fusion of the virus with its target cells (polyanions) have failed to protect against HIV infection. The success of highly active antiretroviral therapy (HAART) provides a paradigm for developing the next generation of microbicides, raising the possibility that a combination of potent and broadly active inhibitors that exhibit multiple and complementary mechanisms of action may be vastly superior to the delivery of single compounds. To fully realize the potential of these potent antiretroviral (ARV) drugs, the challenges of formulating and delivering compounds with markedly different chemical stability and aqueous solubility in a topical combination product must be overcome. This research plan is designed to evaluate nanoparticle-based vaginal drug delivery systems for HIV prevention. The experimental focus is to achieve protection against vaginal transmission of HIV-1 by topical delivery of a combination of antiretroviral drugs using mucus- and tissue-diffusing nanoparticle microbicides. This research would be the first to control the temporal and spatial co-delivery of a combination of antiretroviral agents that have different mechanisms of action against HIV-1 (Aim 1). If successful, our studies would be the first to determine the size range and penetration depth accessible for nanoparticulate drug delivery systems in the vaginal mucosa (Aim 2). Our proposed research will also provide valuable data on the transport, biodistribution, and pharmacokinetics of encapsulated and released antiretroviral agents that are administered topically to the vaginal mucosa using nanoparticle microbicides (Aim 3). Finally, we will conduct preclinical safety and anti-HIV efficacy studies to rapidly advance our nanoparticle-based microbicides to human safety and efficacy trials (Aim 4). The outcomes from our proposed research may highly impact the field of microbicide research for HIV and other sexually-transmitted infections.